The present invention relates to direction finding equipment and in particular to quasi-doppler direction finders especially well suited for maritime purposes.
The portion of the radio band set aside for marine direction finders comprises 155.475 MHz to 162.55 MHz. The allocated band is divided into 284 channels at 25 KHz separation. Accordingly, DF receiver are provided with 25 KHz pass band filters to provide separation between adjacent channels.
In a conventional quasi-doppler DF system, a plurality of dipole antenna elements, in a circular array, commonly 16 elements, are sequentially scanned at 216 Hz, 3.456 KHz dipole to dipole. This generates many side bands of received signals and 3.456 KHz that could lie within the 25 KHz pass band of the DF receiver. As a result, the DF receiver is susceptible to generating bearing errors for a desired channel frequency when an off channel frequency exists simultaneously. This may occur, for example, where a land-based transmitter is located close to the DF receiver or several vessels are simultaneously transmitting from locations close to each other.
It has heretofore been determined that the unwanted side bands result from the modulation process which occurs during the switching from dipole element to dipole element. Accordingly, it is heretofore been proposed to blank out the rise and decay time during the successive switching on and off of each dipole element. Although this reduces the susceptibility for generating bearing errors when a desired channel frequency and an off channel frequency exist simultaneously, it has not been found to be extremely satisfactory since side bands are generated at the leading and trailing edges of the blanking pulses. Of course the problem could be entirely avoided by utilizing a single dipole rotating mechanically but such mechanically rotating systems have other inherent problems which make the quasi-doppler system more attractive.